Re: [PATCH 1/6] sched: decouple PREFERRED_CPU from PARAVIRT in Kconfig.preempt

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi.


On 11/2/26 5:04 PM, Ilya Leoshkevich wrote:
> 
> 
> On 11/2/26 12:00, Peter Zijlstra wrote:
>> On Mon, Nov 02, 2026 at 11:16:30AM +0100, Ilya Leoshkevich wrote:
>>>
>>>
>>> On 11/1/26 20:21, Yury Norov wrote:
>>>> The IBM s390 doesn't enable PARAVIRT config for their hardware
>>>> virtualized Logical Partitions (LPAR). However, the steal time is
>>>> accounted for the LPARs, once the config VIRT_CPU_ACCOUNTING_NATIVE
>>>> is enabled.
>>>>
>>>> Preferred cpumask doesn't rely on any feature gated by PARAVIRT config,
>>>> and requires it to minimize the configuration space and runtime overhead,
>>>> with the lack of non-paravirt user. Now that s390 wants to adopt preferred
>>>> cpumask, there's no reason to keep it coupled to paravirt.
>>>>
>>>> Make PREFERRED_CPU a user-configurable option, even if paravirtualization
>>>> is not enabled, and drop PARAVIRT dependency on the steal governor.
>>>>
>>>> Keep the default module selection conditional on PARAVIRT so other SMP
>>>> configurations can opt in without enabling the governor by default.
>>>>
>>>> Link: https://lore.kernel.org/all/4605dad5-aab2-49e3-b919-bf9897181834@linux.ibm.com/
>>>> CC: Mete Durlu <meted@linux.ibm.com>
>>>> Signed-off-by: Yury Norov <ynorov@nvidia.com>
>>>> ---
>>>> Can be applied independently from the rest of the series.
>>>>
>>>>    drivers/virt/Kconfig   |  8 ++++++--
>>>>    kernel/Kconfig.preempt | 12 ++++++++++--
>>>>    2 files changed, 16 insertions(+), 4 deletions(-)
>>>
>>> This is really useful to us, since even though s390 is nominally
>>> paravirtualized, it does not technically benefit from defining
>>> CONFIG_PARAVIRT.
>>
>> And therefore removing PARAVIRT is the only possible solution?
>>
>>> Specifically, steal time accounting happens by
>>> computing a difference between wall time and CPU time, and not by
>>> explicitly querying the hypervisor.
>>
>> Nobody explicitly asks the hypervisor, that would be horrendously slow.
>>
>> Something trivial like this should do.
>>
>> ---
>>   arch/Kconfig                           | 9 +++++++++
>>   arch/powerpc/platforms/pseries/Kconfig | 1 +
>>   arch/s390/Kconfig                      | 1 +
>>   drivers/virt/Kconfig                   | 2 +-
>>   4 files changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/Kconfig b/arch/Kconfig
>> index 1163736de1f9..8980054beb52 100644
>> --- a/arch/Kconfig
>> +++ b/arch/Kconfig
>> @@ -1080,6 +1080,15 @@ config HAVE_IRQ_TIME_ACCOUNTING
>>   config HAVE_PV_STEAL_CLOCK_GEN
>>       bool
>> +#
>> +#  Archs that can account CPUTIME_STEAL; normal route is through PARAVIRT which
>> +#  selects HAVE_PV_STEAL_CLOCK_GEN. Directly selected by s390 and PPC-LPAR
>> +#  which have their own thing and don't go through generic cputime accounting.
>> +#
>> +config HAVE_CPUTIME_STEAL
>> +    bool
>> +    default y if HAVE_PV_STEAL_CLOCK_GEN
>> +

Can't PARAVIRT_TIME_ACCOUNTING be the kconfig we can rely upon.
s390 can add it to the config (similar to HAVE_CPUTIME_STEAL)?

I guess it is selected currently where the steal time reporting happens.

>>   config HAVE_MOVE_PUD
>>       bool
>>       help
>> diff --git a/arch/powerpc/platforms/pseries/Kconfig b/arch/powerpc/platforms/pseries/Kconfig
>> index 7d125e288f6e..3c49baf606d7 100644
>> --- a/arch/powerpc/platforms/pseries/Kconfig
>> +++ b/arch/powerpc/platforms/pseries/Kconfig
>> @@ -41,6 +41,7 @@ config PPC_SPLPAR
>>       depends on PPC_PSERIES
>>       select PARAVIRT_SPINLOCKS if PPC_QUEUED_SPINLOCKS
>>       select PARAVIRT_TIME_ACCOUNTING if VIRT_CPU_ACCOUNTING_GEN
>> +    select HAVE_CPUTIME_STEAL
>>       default y
>>       help
>>         Enabling this option will make the kernel run more efficiently
>> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
>> index 7138d923d86f..03a884355218 100644
>> --- a/arch/s390/Kconfig
>> +++ b/arch/s390/Kconfig
>> @@ -291,6 +291,7 @@ config S390
>>       select USER_STACKTRACE_SUPPORT
>>       select VDSO_GETRANDOM
>>       select VIRT_CPU_ACCOUNTING
>> +    select HAVE_CPUTIME_STEAL
>>       select VMAP_STACK
>>       select ZONE_DMA
>>       # Note: keep the above list sorted alphabetically
>> diff --git a/drivers/virt/Kconfig b/drivers/virt/Kconfig
>> index eeb84e578ddf..a3721d34a55c 100644
>> --- a/drivers/virt/Kconfig
>> +++ b/drivers/virt/Kconfig
>> @@ -43,7 +43,7 @@ config FSL_HV_MANAGER
>>   config STEAL_GOVERNOR
>>       tristate "Dynamic vCPU management based on steal time"
>> -    depends on PARAVIRT && SMP
>> +    depends on SMP && HAVE_CPUTIME_STEAL
>>       select PREFERRED_CPU
>>       default m
>>       help
> 
> I can't comment on the PPC part, but for s390 this works fine with the
> following addition:
> 
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -197,4 +197,4 @@ config SCHED_CLASS_EXT
> 
>   config PREFERRED_CPU
>          bool
> -       depends on SMP && PARAVIRT
> +       depends on SMP && HAVE_CPUTIME_STEAL

PS: I haven't read the series yet.
Will try to do so when it get little bit of time.